1. Field of the Invention
The present invention relates to communications networks and, more particularly, to a device and method for time synchronization in a communication network, by which an estimation of a pulse counter status of a reference clock is further improved and/or by which a greater number of network nodes may be incorporated in the time synchronization with a predetermined level of accuracy.
2. Description of the Related Art
Communication networks are used in many technical fields to control operating sequences in a decentralized manner. Particularly, in industrial automation systems, it is quite important to precisely coordinate the automatic operating processes with one another. This is achieved by the individual network nodes in the communication network that communicate with one another in each case comprising internal clocks and synchronization messages being transmitted to synchronize all the internal clocks. The internal clock of one respective network node thus operates at a corresponding node clock frequency, which in some instances may be different for the individual network nodes. The clocks are synchronized based on a predetermined reference clock (i.e., “Grandmaster clock”) and/or the reference clock frequency thereof, where the synchronization messages are transmitted according to the reference clock frequency. As a result, synchronization messages are transmitted at fixed clock intervals according to the reference clock frequency.
The individual synchronization messages transmitted in the communication network contain the pulse counter status of the reference clock. Each network node updates this pulse counter status for actual requirements by estimating the number of pulses of the reference clock between the transmission of the synchronization message at the preceding network node and the receipt of the synchronization message at the respective network node (i.e., “line delay”). Moreover, each network node updates this pulse counter status for the next node, by also estimating the number of pulses of the reference clock between the receipt of the synchronization message and the transmission of the synchronization message (i.e., “bridge delay”). Generally, this estimation is performed by estimating, among other things, the pulse ratio (e.g., the “Rate Compensation Factor” (RCF)) between the reference clock frequency and the node clock frequency of the respective network node. Thus, by using the estimated pulse ratio for each desired time interval, the number of pulses of the node clock frequency may be converted into the corresponding number of pulses of the reference clock frequency. Thus, for example, the time interval between the transmission of a synchronization message at the preceding network node and the transmission of the synchronization message at the respective network node, measured in node clock frequency pulses, may be converted into reference clock frequency pulses. The resulting number of pulses is then added to the pulses of the received synchronization message and a correspondingly updated synchronization message is again transmitted from the corresponding network node.
In the field of industrial automation, the International Electrotechnical Commission (IEC) Standard No. 61158 Type 10, i.e., Process Field Network (PROFINET), which is an Ethernet, and which meets industrial requirements is known. This PROFINET Standard operates according to the principle set forth above, according to which the pulse counter statuses in the synchronization messages are updated in the network nodes. Generally, PROFINET-based systems use the Precision Transparent Clock Protocol (PTCP) according to IEC 61158 Type 10 PTCP, which is also referenced as “Profile” in the Institute of Electrical and Electronic Engineers (IEEE) Standard 1588 V2 to synchronize the internal clocks of the network nodes.
This updates the pulse counter statuses of the synchronization messages according to the principle set forth above. According to this standard, synchronization messages are transmitted in succession from one network node to the next in a logical sequence or tree structure. The synchronization messages originate from a reference node and/or master element, which is the first element in the sequence and/or in the tree structure. The synchronization messages originally contain a time stamp of the counter of a reference clock in the reference node, when a synchronization message has been transmitted. The network nodes in the sequence and/or tree structure, i.e., slaves, process and forward this information. A network node thus adds all the estimated time delays between transmission of a synchronization message from the preceding network node and its own transmission of the synchronization message as content to the synchronization message.
A practical implementation is disclosed in R. Lupas Scheiterer, C. Na, D. Obradovic and G. Steindl: “Synchronization Performance of the Precision Time Protocol in Industrial Automation Networks” ISPCS07 Special Issue of the IEEE Transactions on Instrumentation and Measurement, June 2009, Volume 58, Issue 6, pp. 1849-1857.
EP 2034642 A1 discloses a method for transmitting synchronization messages in a communication network, where the communication network comprises a plurality of network nodes communicating with one another, which each contain an internal clock, which is clocked at a node clock frequency assigned to the respective network node. The synchronization messages transmitted in the communication network are used for time synchronization of the internal clocks of the network nodes and contain the pulse counter status (and/or according to the first slave, the estimated pulse counter status) of a reference clock, which operates at a predetermined reference clock frequency. The pulse counter status is estimated by each network node and updated in the synchronization message. When estimating the pulse counter status, alterations in the reference clock frequency are taken into account. An accurate determination of the pulse counter status is achieved by a function, using an approximation of the change over time of the pulse ratio between the reference clock frequency and the node clock frequency, whereby the pulse ratio may be predicted when transmitting a new synchronization message and an accurate updated pulse counter status may be determined based on the predicted pulse ratio.
In conventional methods, an estimated reference clock-pulse counter status is determined, which represents an estimation of a pulse counter status of the reference clock in each network node. The estimated reference clock-pulse counter status is, however, overlaid with noise and does not have a continuous path over the time, because it is re-calculated each time a synchronization message is received. With the requirement, in practice, for accuracy (typically a micro-second) of the estimated reference clock-pulse counter status, the number of network nodes which may be incorporated in the time synchronization is thus strictly limited.